This invention relates to a variable frequency characteristic circuit for a vehicular acoustic device.
Acoustic characteristics inside a vehicle are different from those in the general sound field and have unique patterns because the sound field is surrounded by reflective surfaces such as window glass and a dashboard and by sound absorbing surfaces such as seats. Furthermore, the space of the sound field is narrow unlike the more normal home or office. For instance, the front seats are located roughly in the center of the longitudinal direction of the vehicle with sound sources (speakers) respectively arranged in both front doors. The resultant sound pressures has a frequency characteristic that peaks at about 250 Hz and 1-2 kHz and in which a dip occurs at about 700 Hz.
However excellent are the characteristics that a speaker unit has in an anechoic room, it is impossible to obtain a flat frequency characteristic from such a speaker unit installed in a vehicle. To compensate this disturbance, the frequency characteristics have conventionally been corrected by inserting a variable frequency characteristic circuit in the reproducing system of a vehicular acoustic device. Moreover, a sound quality adjusting circuit for adjusting the sound quality is also normally inserted in the reproducing system of the acoustic device.
FIG. 5 shows a circuit diagram of a conventional acoustic adjusting circuit and a variable frequency characteristic circuit. In FIG. 5, there is shown a buffer amplifier 1 formed with an operational amplifier OP.sub.1 receiving an input signal as a non-inverted input. In a sound quality adjusting circuit 4, a resistor R.sub.1, a variable resistor VR.sub.1, and a capacitor C.sub.1, are connected in series between the output of the operational amplifier OP.sub.1 and ground. A resistor R.sub.2 is connected in parallel across the variable resistor VR.sub.1 Capacitors C.sub.2, and C.sub.3 are respectively connected from both ends of the variable resistor VR.sub.1 and its slider (output). These elements constitute a resonance circuit 2 for adjusting the bass sound quality (low frequency sound). Moreover, a resistor R.sub.3, a capacitor C.sub.4, a variable resistor VR.sub.2, a capacitor C.sub.5 and a resistor R.sub.4 are connected in series between the output of the operational amplifier OP.sub.1 and ground. These elements constitute a resonance circuit 3 for adjusting the treble (high frequency sound). The sliders of variable resistors VR.sub.1 and VR.sub.2 are connected through resistors R.sub.5 and R.sub.6 and the contact common to them is grounded through resistors R.sub.7 and R.sub.8 and a capacitor C.sub.6. The contact common to the resistors R.sub.7 and R.sub.8 is connected to the inverted input of the operational amplifier OP.sub.1. The sound quality of the bass and treble are respectively adjusted by the variable resistors VR.sub.1 and VR.sub.2 in the sound quality adjusting circuit 4 thus arranged.
The output signal of the sound quality adjusting circuit 4 is applied through a resistor R.sub.9 to the non-inverted input of another operational amplifier OP.sub.2. The operational amplifier OP.sub.2 and a feedback resistor R.sub.10 connected between the output and inverted input of the operational amplifier OP.sub.2 constitute a buffer amplifier 5. Resonance circuits 6 and 7 whose resonance frequencies are different from each other are connected between the non-inverted input of the operational amplifier OP.sub.2 and ground through damping resistors R.sub.11 and R.sub.12, whereas a resonance circuit 8 is connected between the inverted input of the operational amplifier OP.sub.2 and ground through a damping resistor R.sub.13. There results a variable frequency characteristic variable circuit 9 for correcting the transmission frequency characteristics inside a vehicle.
Since the sound quality adjusting circuit 4 and the frequency characteristic variable circuit 9 each have their own separate buffer amplifiers, as described above, the combined circuit is disadvantageous in being not only costly but also handicapped in view of acoustic performance. That is the signal-to-noise ratio (S/N) and the distortion factor are degraded because two buffer amplifiers are inserted on the signal line.